1. Field of the Invention
The present invention relates to integrated circuits and in particular to an inductive element of an integrated circuit, comprising at least one turn, which is formed by an integrated elongated track made of a conductive material.
2. Description of the State of the Art
Such elements are known in particular from the field of integrated high-frequency circuits where they are also known as xe2x80x9cintegrated coilsxe2x80x9d or xe2x80x9cmonolithic coilsxe2x80x9d. Such coils can be made using any of the usual production technologies applying to integrated circuits. Advantageously, in such production of coils on or in a substrate of an integrated high-frequency circuit, instances of undesirably large electrical and magnetic coupling can be avoided, as they would necessarily occur with the use of external coils, for example in the field of contact pads and bond lines.
Inductivity and electrical resistance are important parameters of an inductive element. Together with the electrical characteristics of the direct surroundings of the element, the ratio of these two parameters determines the so-called quality, or quality factor, as well as the resonance frequency of the element or of a resonating circuit based on such an element. Space requirement is a further important parameter of an integrated inductive element.
Although these parameters greatly depend on the layout and the characteristics of the production technology used, relatively narrow boundaries always exist in relation to these parameters in the design of an integrated inductive element. Often, the element is associated with considerable resistance, because the width of the conductive track is limited by the space available, on a semiconductor substrate, for the element, and because the height of the conductive track is limited by the production technology used. For a given available space, the maximum achievable inductivity is considerably limited, in particular because in the case of an integrated circuit the arrangement of a multitude of turns (ring-shaped conductor sections) essentially has to be in two dimensions (e.g. in one plane of the layout). If the resistance of the conductive track is not to be excessive, only a few turns can be accommodated in a given surface area.
In summary, in the design of integrated inductive elements, there are conflicting objectives concerning the important parameters. The implementation of such an element which has modest space requirements, low resistance and high inductivity poses particular problems.
It is thus the object of the present invention to provide an inductive element of an integrated circuit of the type mentioned in the introduction in which there is greater freedom in selecting the parameters.
This object is met by an inductive element comprising at least one recess. The dependent claims relate to advantageous improvements of the invention, which may of course be combined with one another.
The invention provides for the interior margin of the conductive track of the inductive element to comprise at least one recess. In this way, it is possible, for example with a predetermined space requirement and a predetermined resistance, to increase the inductivity; or with a predetermined resistance and a predetermined inductivity, to save space. Irrespective of this, there is a reduction in the capacitive coupling to the substrate.
The element can be implemented using various production technologies (CMOS, bipolar, BICMOS, GaAS, etc.). In order to provide an element with low resistance, the material of the conductive track can be a metal. As an alternative, heavily doped semiconductor material or polysilicon can also be considered.
Preferably, the arrangement of the conductive track is essentially polygonal (e.g. rectangular) or circular in shape. With a predetermined inductivity and width of the conductive track, particularly low resistance can be achieved with a circular arrangement of the conductive track.
In a preferred embodiment, the conductive track is arranged along at least 80% of the circumference in the shape of a rectangle or a circle. For high-frequency circuits featuring high integration density, which circuits are of particular interest in the context of this invention, a uniform width of the conductive track or a local maximum width of the conductive track is less than 100 xcexcm, in particular less than 50 xcexcm. If the inductive element comprises only one turn, then the ratio of width of the conductive track (averaged over the length of the conductive track) to maximum transverse extension of the turn preferably ranges between 0.01 and 0.4.
Preferably, the conductive track (apart from its recesses) is of essentially uniform width, in particular a width which along the entire length of the conductive track at most deviates by 20% from the average width of the conductive track. The improvement in the characteristics of the element is particularly good if the extent of the recess in transverse direction of the conductive track is in the range of 0.1 times to 0.5 times the width of the conductive track, and/or if the extent of the recess in longitudinal direction of the conductive track is in the range of 0.05 times to 0.5 times the width of the conductive track.
The extent of a recess in longitudinal direction of the conductive track should be selected at least so that during operation of the inductive element with electrical loads according to the specifications of this element, no breakdown due to excessive electrical field strength occurs in the region of the recess. To prevent breakdown, a preferred embodiment provides for the use of a material in the area of the recess, which material differs from the substrate material of the integrated circuit and provides better breakdown strength than does said substrate material. For example, the inductive element can be formed in the uppermost conductor plane of an IC, which is covered by an insulating passivation layer, which also extends into the recesses.
Generally speaking, particularly good characteristics of the element can also be achieved if the interior margin of the conductive track comprises several recesses (e.g. notches) which in longitudinal direction of the conductive track are essentially evenly spaced apart. In this arrangement it has been shown to be advantageous if the sum of the extent of the recesses in longitudinal direction of the conductive track ranges from 0.05 times to 0.2 times the length of the conductive track.
In particular, if the width of the conductive track in the region of a recess is relatively small, e.g. within the magnitude of the structural width which can be achieved with the production technologies in use, a recess of rectangular shape is advantageous. With regard to the electrical characteristics of the element, a recess in the shape of an indentation with a curved indentation margin is favourable, e.g. a semicircular indentation.
Preferably, the ratio of the recess area to the area of the conductive track (with an imaginary continuation of the conductive track in the region of the recesses) ranges from 0.005 to 0.25.